Today's commercially employed thermoset material systems usually have maximum glass transition temperatures of 180° C. and are described as epoxy resins. However, glass transition temperatures of 180° C. are too low for many applications, particularly under high thermal and mechanical loads. In contrast, other commercially available materials for high temperature applications are often toxic (for example BMI), very expensive and/or difficult to process. Cyanate esters, on the other hand, have low viscosities at low temperatures, which means that it is possible to use these thermosets in injection processes (such as RTM, VAP etc.). In addition, compared with other high temperature-resistant thermosets, they are inexpensive and commercially available. One disadvantage of cyanate esters, however, is that because of the high cross-linking density of the triazine network, they are usually very brittle. In addition, the triazine network can be sensitive to moisture. In order to be able to use cyanate esters in aviation, then, suitably appropriate modifications or blends have to be produced.
Polyimides are very expensive, which usually limits the use of these thermosets to highly specialized applications and demanding situations. What is more, polyimides have high melting temperatures and at the same time have very high viscosities, and so frequently, they can only be processed using special equipment and techniques. However, polyimides are distinguished by an extraordinarily high thermal stability with simultaneous high mechanical strength. In order to be able to process polyimides into matrix resins for fibre-reinforced composites, however, appropriate modifications have to be carried out.
Many cyanate ester (CE)/polyimide combinations are known. As an example, polyimides have recently been used primarily in the form of thermoplasts in order to modify the impact strength of brittle cyanate esters; see, for example, T. Iijima, T. Tomohiro and M. Tomoi, Journal of Applied Polymer Science, 2003, 88, 1-11 or M. DiBerardino, Dissertation, 1993, Lehigh University. U.S. Pat. No. 4,370,462 describes the modification of a cyanate ester system using an ethynyl-terminated polyimide and an amine. In that document, the cyanate ester is initially reacted with the ethynyl-terminated polyimide in a pre-reaction, whereupon a mixture which is soluble in N-methylpyrrolidone is formed. It is assumed therein that the nucleophilic nitrogen of the cyanate ester reacts with the π-bond of the terminal ethynyl via a Michael addition reaction. In the second step, the product which is formed is dissolved in N-methylpyrrolidone and cross-linked with the aid of a transition metal catalyst and a polyfunctional amine. The end group of the polyimide used is exclusively limited to ethynyl. Furthermore, the development of the thermosetting interpenetrating network starting from cyanate esters and aryl ethynyl-terminated polyimides which are described in the present invention is governed by a completely different mechanism to that described for the formation of the thermoset network described in U.S. Pat. No. 4,370,462.
I. Hamerton, High Performance Polymers, 1996, 8, 83-95; S. P. Qureshi, U.S. Pat. No. 4,774,282; J. Fan, X. Hu and C. Y. Yue, Journal of Polymer Science: Part B: Polymer Physics, 2003, 41, 1123-1134; A. Gu, Composites Science and Technology, 2006, 66, 1749-1755; I. Hamerton, J. M. Bartona, A. Chaplinb, B. J. Howlina and S. J. Shawb, Polymer, 2001, 42, 2307-2319 and C. Gouri, C. P. Reghunadhan Nair, R. Ramaswamy and K. N. Ninan, European Polymer Journal, 2002, 38, 503-510 describe the covalent bonding of an imide-containing polymer and a cyanate ester using the example of BT (bismaleimide/triazine) resins. J. M. Barton, I. Hamerton and J. R. Jones, Polymer International, 1992, 29, 145-156 describe that preferably, a compatibilizer is added to the thermoset blend since a covalent bond between a cyanate ester and a bismaleimide network cannot be formed directly. The constitution and the chemical/physical characterization as well as the invenstigation of the mechanical properties of the blend consisting of cyanate ester/bismaleimide/DABPA has already been investigated in detail; see U.S. Pat. No. 4,774,282; C. Gouri, C. P. Reghunadhan Nair, R. Ramaswamy and K. N. Ninan, European Polymer Journal, 2002, 38, 503-510; G. Anuradha and M. Sarojadevi, High Performance Polymers, 2006, 18, 1003-1018 and X. Hu, J. Fan and C. Y. Yue, Journal of Applied Polymer Science, 2001, 80, 2437-2445. Furthermore, I. Hamerton, J. M. Bartona, A. Chaplinb, B. J. Howlina and S. J. Shawb, Polymer, 2001, 42, 2307-2319 describe the constitution of an alkenyl-functionalized aryl cyanate ester which is capable of reacting with a commercial cyanate ester resin and simultaneously, via the allyl function, with the π-bond of the bismaleimide.
In general, cross-linkable polyimides perform very well as regards thermal resistance as well as mechanical strength; see Yanfeng Liu, Then Wang, Gao Li and M. Ding, High Performance Polymers, 2010, 22, 95-108 and M. Miyauchi, Y. Ishida, T. Ogasawara and R. Yokota, Reactive and Functional Polymers, 2013, 73, 340-345. Thus, this type of thermoset is potentially suitable for modifying the impact strength of brittle resin systems such as cyanate esters, for example, without loss of thermo-mechanical properties such as the Tg as well as the thermal capacity.